Method of surface-treating metals



METHOD OF SURFACE-TREATING METALS Hideya Okada, Tokyo, Japan, assignorto Yawata Iron & Steel Co., Ltd., Tokyo, Japan N Drawing.Continuation-impart of application Ser. No.

357,726, Apr. 6, 1964. This application Dec. 20, 1967,

Ser. No. 691,925

Int. Cl. C23b 5/50, 5/56 U.S. Cl. 204-41 3 Claims ABSTRACT OF THEDISCLOSURE This application is a continuation-in-part of copendingapplication Ser. No. 357,726, now abandoned.

This invention relates to methods of surface-treating metals and moreparticularly to methods of surface-treating iron and steel.

Generally chromium plating has various advantages, e.g., it is pleasingin appearance, does not discolor in the atmosphere and is high inheatproofness, hardness and wear-resistance. Therefore, the field of itsapplication is expanding more and more. But it has a defect in that thechromium film is so likely to produce pores or cracks that the groundmetal can not be perfectly coated. It is therefore usual to apply copperor nickel plating as an intermediate layer onto the ground metal to bechromium plated and then to apply a very thin chromium plating thereto.However, even with such a chromium plating method, the anticorrosivenessis far from being perfect.

According to the present invention, by using a chromate film as a groundwithout applying such copper or nickel plating as is used as a groundfor conventional chromium plating, there can be obtained a protectivefilm which shows excellent anticorrosiveness even under very severeconditions and is high in workability. That is to say, the presentinvention is directed to a method of treating metal surfacescharacterized by applying a chromate film onto the metal surface to betreated and then chromium-plating said chromate film.

Chromate treatment is used to prevent rust and has a wide range ofapplicability. Such film is composed of an oxide of chromium, has asubstantially amorphous structure and is therefore very high inanticorrosiveness and workability. By using it as a ground for chromiumplating, the anticorrosiveness and workability of the chromuim-platedfilm can be remarkably improved.

In the conventional methods that, for example, use nickel plating as aground for chromium plating, generally the nickel layer used as theground will be exposed through cracks in the chromium plating.Therefore, when the chromium plating is left in a corroding environment,a potential difference will be produced between chromium plating andnickel plating and corrosion will be accelerated. On the other hand,according to the method of the present invention, a chromate film existsas a ground for chromium plating and therefore, even though a chromatefilm is exposed through cracks in the chromium plating, no potentialdifference will be produced between the chromate film and chromiumplating and therefore corrosion will be remarkably prevented.

United States Patent 0 Such adhesion between the chromate film andchromium plated layer as is obtained by the method of the presentinvention is so high as to endure severe working. In addition, theplating of the present invention is so high in paint adhesiveness as tobe able to be advantangeously used as a ground for painting.

In the method of the present invention, the surface of the metal to betreated is first degreased by any normal process, is pickled and is thencoated with a chromate film thereon. The chromate treatment itself canbe done with any known process. The chromate film can be applied ontothe surface of the metal to be treated, for example, by carrying out anelectrolysis with the metal as the cathode in an aqueous solutioncontaining chromic acid or by dipping the metal in an aqueous solutionof chromic acid containing a reducing agent, squeezing it with rolls andthen heating and baking it. For example, in the case ofchromate-treating a steel sheet by the former process, the electrolysismay be carried out with the steel sheet as a cathode in an aqueoussolution containing about 0.1 to 3 moles of pure chromic acid. It ispreferable that the current density is about 15 to 100 a./dm. theelectrolyzing time is about 1 to 60 seconds, and preferably 1 to 5seconds, the bath temperature is about 20 to 50 C. and the thickness ofthe chromate film is about 100 to 5000 A. However, depending on thecircumstances, other conditions then are mentioned above can be used. Insome cases, the chromate-treating bath to be used may be prepared byadding such chromate film production accelerators as, for example,selenic acid, potassium zirconium fluoride, potassium titanium fluorideor perchloric acid to an aqueous solution of chromic acid.

The typical bath composition in the case of treating, for example, asteel sheet with a bath prepared by adding a reducing agent to anaqueous solution of chromic acid in the latter process is about 15%chromic acid, about 6% zinc oxide, about 15% ammonium molybdate andabout 5% cane sugar, the rest being water. When the steel sheet isdipped in this bath, is squeezed with rolls and is then heated at atemperature of about 550 C. for about 1 minute, a chromate film Will beproduced on the surface of the steel sheet.

Needless to say, the process for applying a chromate film onto thesurface of a metal according to the method of the present invention isnot limited to the above mentioned process but may be any otherchromate-treating process.

The metal onto which a chromate film has been applied as in the above isthen chromium plated in the second step of the present invention. Thischromium plating step itself can be applied by any known process.Specifically, it is preferred to carry out the chromium plating by usinga current density of about 20 to 60 a./dm. at a bath temperature ofabout 45 C. for about 1 to 60 seconds in an ordinary Sargent bath (of lto 3 mols of CrO and 0.01 to 0.03 mole of H The specifically preferablerange is a current density of 25 to 35 a./dm. and a treating time of lto 5 seconds. The thickness of the chromium-plated layer may preferablybe about to 5000 A. However, when applying the present method ontin-free steel, a total thicknes of between 100 and 500 A for both thechromate film on the chrome plating layer is preferred.

Needless to say, the method of the present invention can be applied tosuch final metal product. Further, the chromium-plated layer obtained bythe method of the present invention is so high in anticorrosiveness,workability and paint adhesiveness as shown by the data below that itcan be advantageously applied to such material as a metal sheet.

The method of the present invention shall be explained with reference toexamples in which it is applied to nickel-plated surface of steelsheets. However, the subject matter of the present invention is notlimited to such application but can be applied to such variousmetal-plated surfaces as zinc, iron and copper.

EXAMPLE 1 A mild steel sheet was degreased and pickled and was thenfirst treated under the below mentioned conditions in a chromate bath toapply a chromate film onto it.

Solution0.5 mol of CrO Electrolyzing conditions:

Current density30 at/din. Temperature30 C. Treating time seconds Thesteel sheet having the thus obtained chromate film was chromium platedunder the below mentioned electrolyzing conditions in the belowmentioned plating bath. The chromium-plated surface on the steel sheetobtained after water-rinsing and drying showed a high anticorrosiveness,workability and paint adhesiveness.

Chromium plating bath 3 mols of CrO +0.03 mol of H 80 Electrolyzingconditions:

Current densitya./dm. Temperature- C. Treating time30 seconds Example 2A mild steel sheet was degreased and pickled and then a chromate filmwas applied onto said steel sheet under the below mentionedelectrolyzing conditions in the below mentioned chromate bath.

Chromate bath-0.5 mol of CrO +0.5 g./l. of H SeO Temperature-45 C.Treating time-30 seconds When the steel sheet having the thus obtainedchromate film was chromium-plated under the chromium-plating conditionsshown in Example 1, the chromium-plated surface showed properties highin anticorrosiveness, workability and paint adhesiveness.

Example 3 The plated surface of a steel sheet nickel-plated by a knowntreating process by using a Watts bath was cleaned, was thenchromium-treated as shown in Example 1 and was chromium-plated on thechromate film under the same conditions as in Example 1 except that thetreating time was 1 minute. The thus obtained chromium-plated surfaceshowed the same very high anticorrosiveness, workability and paintadhesiveness as in Examples 1 and 2.

The results of the anticorrosiveness tests of the chromium-plated layerson the mild steel sheets as obtained by the method of the presentinvention and conventional methods are shown in Table 1.

TABLE l.ItESULTS OF ANTICORROSIVENESS TESTS Sample No. N-30: Thetreating conditions were as mentioned in Example 2.

Sample No. N-31: The plating was made for 1 minute under the conditionsin Example 1.

Sample No. N-33: The nickel plating was made in a Watts bath containinga brightener for 30 seconds at a current density of 10 a./dm. Thechromium plating was made under the same conditions as in Example 2.

The results obtained with the mentioned respective samples after saltspray tests for 24 hours are shown in Table I. It will be seen that theanticorrosiveness of sample No. N-30 as obtained by the method of thepresent invention is much higher than of the Samples Nos. N-31 and N-33by conventional methods.

Further, even when the sample by the method of the present invention wassubjected to an outdoor exposing test, no rust was produced for severalmonths.

The results of the workability test and paint adhesiveness test of thesample obtained by the method of the present invention are shown inTable 2.

TABLE 2RESULTS OF WORKABILITY AND PAINT ADHESIVENESS TESTS Paintadhesiveness test (melamine alkyd paint) workability test (EricsenPicture drawing Sample N 0. test) Du Pont test test Sample obtained N opeeling was No painted N o painted by Example 2. caused at all filmpeeled film peeled by extrusion oil. oil". of 7 mm.

EXAMPLE 4 A mild steel sheet was degreased and pickled and was thenfirst treated under the below mentioned conditions in a chromate bath toapply a chromate film onto the surface thereof.

Bath composition-C10 0.5 mol Bath temperature-45 C. Current density50a./dm. Time--1 second The steel sheet having the thus obtained chromatefilm was chromium plated under the below mentioned electrolyzingconditions in the below mentioned plating bath.

Bath composition-CrO 2.5 mol; H 2.5 g./l. Bath temperature45 C.

Current density50 a./dm.

Time-4 seconds The chromium plated steel sheet obtained by the abovemethod showed a beautiful metallic luster in appearance. Itsanticorrosive properties in a salt water spray test for 6 hours was only10%. Further, even when the chromium plated steel sheet was dipped in 1%sulfuric acid for 24 hours, only 40,41. g./ml. of iron was dissolved.

EXAMPLE 5 A mild steel sheet was degreased and pickled and was thenfirst treated under the below mentioned conditions in a chromate bath toapply a chromate film onto it.

Bath compositionCrO 0.5 mol; H 80 1.2 g./l.

Bath temperature-45 C.

Current density50 a./dm.

Time3 seconds m l The steel sheet having the thus obtained chromate filmwas chromium plated under the below mentioned electrolyzing conditionsin the below mentioned plating bath.

Bath compositionCrO 2.5 mol; H 50 2.5 g./l. Bath temperature-45 C.

Current density-50 a./dm.

Time2 seconds 6 The film obtained by the above method showed a beaubeobtained very quickly by the method of the present tiful metallic lusterin the appearance and, had excellent invention. anticorrosiveproperties. For example, when the steel sheet The example belowillustrates that the appearance or was subjected to a salt water spraytest for 6 hours, no luster of the chromium plated metal surface dependson rust was produced and, when the chromium plated steel the conditionsutilized in producing the chromium film sheet was dipped in 1% sulfuricacid for 24 hours, only 80 g./ml. of iron was dissolved.

prior to the plating operation.

EXAMPLE 8 The following examples lllustrate the fact that when a groundmetal is first coated with a chromium film and A m1ld Steel Sheet wasdegreased and Plckled w subsequently plated with chromium, a chromiumplated then first treated under the below mentioned conditions groundmetal having an excellent metallic luster can be in a chromate bath toapply a chromate film Onto obtained in a much shorter time than if nochromium B th c0rnp0siti0n-CrO 0.5 mol; H 80 0.5 to 1.2 g./l. film isformed on the surface of the metal prior to the B h tem erature 4j C,

chromium Plating 0f Said ground metal; Current density50 a./dm. with Pbanode Electrolyzing time2 to 3 seconds EXAMPLE 6 The steel sheet wasthen chromium plated according to the procedure and conditions as shownin Example 6 A m1ld steel sheet was degreased and pickled and was above.

then first treated under the below mentioned conditions h electrolyzingi f h h i 1 i in in a chromate bath to pp a chromate Plating On the ondsand the variation of the appearance may be seen in surface thereof. thetable below.

Electrolyzing time of chromium-plating in seconds 1 2 3 4 5 10.

Appearance No metallic No metallic There was There was There was Therewas luster. luster. a metallic a metallic a metallic a metallic luster.luster. luster. luster. Reflection rate 98 98 85 80.

Bath composition-CrO 2.5 mol; H 80 2.5 g./l. Bath temperature45 C.Current density-50 a./dm.? with Pb anode It should be noted that whenselenic acid is added to the bath composition for making the chromatefilm, a metallic luster will be shown more quickly during the Theappearance of the metal thus treated may be seen electroplatingoperation. in the table below: The fact that a chromate layer is thuselectrodeposited Electrolyzing time in seconds 1 3 .2 5 7 10.

Appearance N0 metallic No metallic No metallic No metallic There wasluster. luster. luster. luster. a metalic luster. Reflection rate 85.

As can be seen by the above showings, it requires at below and thenchromium-plating is made is epochal as least 10 seconds for a chromatefilm to be plated on the a method of not only increasing theanticorrosiveness but surface of the stainless steel sheet when nochromium film also obtaining an appearance having a metallic luster isfirst filmed on the surface of the steel sheet. within a very shorttime. In case a chromium-plating bath of 250 to 300 g./l. of CrO +2.5 to3.0 g./l. of H EXAMPLE 7 which is most generally used is used, unless amethod of This example demonstrates the variance of appearance making achromate layer below is used, it will be nearly of a metal sheet, when ametal sheet is treated according 5O impossible to obtain on iron orsteel an appearance having to the method of the present invention. ametallic luster within a few seconds.

A mild steel sheet was degreased and pickled and was However, once achromium-plating bath of a low conthen first treated under the belowmentioned conditions centration is used, that is, a bath compositioncontaining in a chromate bath to apply a chromate film onto it. 100 to150 g./l. of CrO is used, an appearance having a Bath composition cros,0.5 mol metallic luster will be able to be obtained within a com- Bathtemperature paratrvely short t1me. However, m such case, as 1s wellCurrent ajdmz known, the anticorrosiveness Wlll be reduced to as muchElectrolyzing to 2 seconds lower value than in the case of using a bathcomposltion having 250 to 300 g./l. of CrO as a main ingredient.

The steel sheet was then chromium plated according to Therefore, even ifa chromate layer is made below, in

the procedure and conditions shown in Example 6 abovecase the bathcomposition of the chromium plating bath The appearance of the metalthus obtained may be to be electrodeposited thereon has to g./l. of

seen in the table below: CrO as a main ingredient, the anticorrosivenesswill be Thus, as can be seen above, in the case wherein a 79 low. Evenin the case there is a chromate layer below, it

chromate layer is produced below, a plated surface having will bepreferable to use a bath composition having 250 to a metallic luster canbe obtained very quickly. Even if 300 g./l. of CrO as a main ingredientfor the chromiumthe time required to make the chromate layer and theplating bath to be electrodeposited thereon.

time for electrodepositing the chromium-plated layer are From the above,it may be seen that in making a added together, it can be seen that ametallic luster may 75 product high in anticorrosiveness and metallicluster on an iron or steel plate, it is preferable to first make achromate layer on iron or steel and then electrodeposit chromium from abath composition having 250 to 300 g./l. of CrO as a main ingredient. Insuch case, it is considered best to electrodeposit a chromate layerunder the conditions of a bath temperature of 30 to 50 0, currentdensity of 20 to 70 a./dm. and electrolyzing time of 1 to 3 seconds froma bath composition having 25 to 70 g./l. of chromic acid as a mainingredient (to which 0.1 to 5 g./l. of selenic acid may be added) andthen electrodeposit chromium under the conditions of a bath temperatureof 30 to 45 C., current density of 20 to 70 a./dm. and electrolyzingtime of 1 to 4 seconds from a bath containing a bath composition having250 to 300 g./l. of chromic acid as a main ingredient and a small amountof an additive (such as a sulfuric acid radical).

As pointed out above, the concentration of the chrmium plating bath ispreferably 250 g./l. of CrO For example, when a steel sheet iselectroplated for 15 seconds with 250 g./l. of CrO and another is platedwith 100 g./l. of CrO for 15 seconds and both are subjected to a saltwater spraying test, the following diiferences in rusting may be shownbelow:

Rusting in salt water spray test for 24 hours: Percent 100 g./l. of CrO1 g./l. of H 50 70 250 g./l. Of CrO 2.5 g./l. Of H SO 10 What is claimedis:

1. A method of surface-treating metals comprising electrolyticallytreating an iron or steel product as a cathode in a chromate-treatingbath having chromic acid as a main ingredient so that a film consistingof chromium oxide may be deposited on the surface of said iron or steelproduct and then cathode-electrolytically treating it in achromium-plating bath containing sulfuric acid for 5 seconds at most ata current density of about 2060 a./dm. so that a chromium-plated coatinghaving a metallic luster may be formed on the surface of the iron orsteel product through the above mentioned chromate film within a veryshort time.

2. A method of surface-treating metals comprising electrolyticallytreating an iron or steel product as a cathode in a chromate-treatingbath containing chromic acid as a main ingredient therein, at a currentdensity of about 15 to 100 a./dm. so that a film consisting of chromiumoxide may be deposited on the surface of said iron or steel product, andthen cathode-electrolytically treating it in a chromium-plating bathconsisting of an aqueous solution of 2.5 to 3.0 g./l. of sulfuric acidand 250 to 300 g./1. of chromic acid for 1 to 5 seconds at a currentdensity of about 20-60 a./dm. so that a chromiumplated coating having ametallic luster may be formed on the surface of the iron or steelproduct through the above mentioned chromate film within a very shorttime.

3. A method of surface-treating metals comprising electrolyticallytreating an iron or steel product as a cathode at a current density of20 to 70 a./dm. for 3 seconds at most in a chromate-treating bath at abath temperature of 30 to C. having 25 to 70 g./l. of chromic acid as amain ingredient and containing 0.1 to 5 g. /l. of selenic acid so that avery thin chromate film may be deposited on the surface of said iron orsteel product and then cathode-electrolytically treating it in achromium-plating bath consisting of an aqueous solution of 2.5 to 3.0g./l. of sulfuric acid and 250 to 300 g./l. of chromic acid for 1 to 5seconds at a current density of 20 to a./dm. so that a chromium-platingcoating having a metallic luster may be formed on the surface of theiron or steel product through the above mentioned chromate film within avery short time.

References Cited UNITED STATES PATENTS 2,603,593 7/ 1952 Blickensderfer20416 2,812,297 11/1957 Stareck et a1. 20434 3,081,238 3/1963 Gurry20434 3,288,691 11/1966 Yonezaki et a1 20456 3,316,160 4/1967 Uchida eta1. 20441 ROBERT K. MIHALEK, Primary Examiner W. B. VAN SISE, AssistantExaminer US. Cl. X.R. 20429, 36

